Dental caries is a disease manifested by local demineralization of the hard tissues of the tooth crown induced by dental plaque. The demineralization process progresses from the outer enamel surface of the crown through the entire thickness of the enamel and into the dentine. Caries lesions of occlusal, buccal and lingual (palatinal) surfaces can be diagnosed by mechanical probing and/or visual inspection. On the other hand, small and medium size lesions of interproximal crown surfaces are hidden by the gingiva and adjacent teeth and have hitherto been identifiable only on radiographs. Although the use of bitewing radiographs is well accepted as an important adjunct in the diagnosis of proximal caries lesions, this method exhibits several weaknesses related to its relative insensitivity and user dependence in terms of technical performance and interpretation [Waggoner W., F. Crall J. J. (1984) Quintessence International 11/1984: 1163-1173]. Furthermore, bitewing radiographs comprise a high proportion of x-rays taken in the dental office. This is in contrast with the current trends in safety standards which support every effort aimed at reducing the exposure to ionizing irradiation. In addition, an alternative technology for the detection of interproximal caries is expected to reduce the environmental pollution and cost associated with the use of x-ray technology.
Caries lesions not adjacent to a dental restoration on a tooth surface site are known as primary caries, while caries lesions in contact with a dental restoration at the tooth surface are known as secondary caries.
The potential of ultrasonic technology for the detection of dental caries, has been proposed in several instances. It has been shown that the hard tissues of the tooth crown, in particular the outer enamel layer, are highly uniform in their sonic properties among different teeth and individuals [Ng S. Y., Payne P. A., Cartledge N. A., Ferguson M. W., (1989), Arch. Oral Biol. 34: 341-345; Barber F. E., Lees S., Lobene R. R., (1969), Arch. Oral Biol. 14: 745-760]. Using a longitudinal ultrasonic irradiation, a specific profile of ultrasonic echoes is obtained from the enamel surface, dentinoenamel junction and pulpodentinal junction. Changes in this profile have been described in instances of demineralization lesions indicating a substantial difference in the sonic conductivity between sound and dimineralized enamel [Ng S. Y., Ferguson M. W. J., Payne P. A., Slater P., (1988), J. Dent 16:201-209; PCT no. WO 95/04506]. These changes result from conversion of the intact enamel to the water rich demineralized material. However, the detection of these lesions is dependent on a direct contact between the ultrasonic probe and the demineralized enamel; such contact cannot be formed in interproximal sites. In addition, echo profiles to longitudinal waves obtained from sound and demineralized tooth crowns are complicated and can be analyzed only by using complex systems.
The present invention relates to a revolutionary approach regarding the use of ultrasonic technology for the detection of primary and secondary dental caries, as well as for the detection of tooth crown surface cracks. Rather than transmitting longitudinal ultrasonic waves into the tooth, as in the prior art, the present invention relates to a probe comprising an ultrasonic surface wave generator that imparts surface ultrasonic waves onto the tooth crown surface, the surface waves then migrating along same. Surface or Rayleigh waves are well known [Cook E. G., Van Valkenburg H. E., (1954) ASTM-Bull 84]. Surface waves migrate uninterruptedly on smooth, flat or curved, contours. Sharp angles, interferences and interfaces present on the surface produce distinct echoes [Krautkramer H., Ultrasonic Testing of Materials, (1969) Springer-Verlag Berlin Heidelberg New York; pp. 257-271]. The amplitude and shape of these echoes or reflections are dependent on the geometry of such interferences. Thus, the interface between a caries lesion or a tooth crown surface crack and intact enamel may be identified by an echo or reflection of surface ultrasonic waves produced thereat and received by a suitable ultrasonic surface wave receiver, typically unitary with said ultrasonic surface wave generator. Since said reflected surface ultrasonic waves have an amplitude substantially greater than the general level of background noise, the profiles of said waves are relatively simple to analyse. Furthermore, since the ultrasonic probe of the present invention does not require to be placed directly onto the zone of interest on the tooth surface, it is particularly useful for the detection of caries lesions and tooth crown surface cracks in areas such as the interproximal site, hitherto inaccessible with ultrasonic probes or devices of the prior art.
The ultrasonic surface wave generator of the present invention comprises any ultrasonic probe capable of transmitting ultrasonic surface waves along a tooth surface. Such an ultrasonic probe may take the form, for example, of an ultrasonic transducer in which the crystal face of the transducer is substantially at an angle B, substantially different from 0.degree., to the tested surface, i.e., the tooth crown surface, by virtue of an appropriately designed wedge-like coupler having a contact surface for abutting against the tested surface. Angle B is known as the overall wedge angle of the coupler. In particular, angle B is substantially greater than 0.degree. and less than 90.degree.. The wedge-like coupler enables substantially longitudinal ultrasonic waves generated by the transducer to be imparted on the tooth surface as ultrasonic surface waves when the wedge-like coupler is in contact with a tooth surface via the contact surface of the coupler.
In particular, the ultrasonic waves generated by the transducer may be focused to achieve a higher sensitivity and resolution. The term "focus" is generally understood to mean a concentration of the ultrasonic beam generated by the transducer to a size less in diameter than the diameter of the piezoelectric crystal face. A circular flat piezoelectric crystal or oscillator may have a quasi-focus at the end of the near field, this being a natural one produced by diffraction phenomena. Nonetheless, the term "focus" herein refers rather to the further focusing, either full or partial, of ultrasonic sound waves to an extent substantially greater than is normally achieved from natural diffraction effects obtained with an isolated but similar non-focused transducer having a planar distal face. In particular, the term "focusing" refers to such further focusing arising from at least one focusing element. The focusing element may be optionally integral with the transducer, and may consist, for example, of the distal face of the focused transducer, the distal face being substantially concave with a concavity having an overall curvature substantially sufficient to enable ultrasonic sound waves generated by the focused transducer to be at least partially focused. Alternatively or additionally, the focusing element may constitute separate from the transducer, comprising suitable focusing means such as suitable lenses, mirrors and/or phase plates.
There are two general categories of focusing transducers: spherically focused transducers provide a spot focused beam, while cylindrical focused transducers provide a line focused beam. Spherically focused transducers generally enhance sensitivity as compared with unfocused transducers, and therefore have the advantage of enabling the existence of even small lesions to be detected when incorporated in the probe or device of the present invention. Cylindrical focused transducers generally provide better resolution as compared with unfocused transducers, and thus enable different adjacent lesions to be identified when incorporated in the probe or device of the present invention.
With focused transducers, the overall wedge angle B of the coupler may be conveniently defined as the angle between a plane substantially perpendicular to the principal axis of the focusing element of the focused transducer, and a plane substantially tangential to the contact surface of the coupler at the point of intersection of the principal axis with the contact surface, the wedge angle B being substantially different to 0.degree., and in particular greater than 0.degree. and less than 90.degree..
The present invention is defined by the claims, the contents of which are to be read as included within the disclosure of the specification, and will now be described by way of example with reference to the accompanying Figures.